Time synchronization is a very important issue for mobile communication systems, especially for TD-SCDMA, WiMAX or LTE systems. Generally these communication systems need time accuracy within several microseconds (μs).
GPS (Global Positioning System) has been widely used by a verity of devices to get an accurate time clock, and now is used for time synchronization for RBS (Radio Base Station). It is a satellite based Global Positioning System developed by the US Department of Defense.
But the application of GPS system is limited by its satellite communication mechanism. For example, the GPS system has difficulty in site selection, especially for in-door RBS, where the satellite signal from the GPS system is too weak to be detected. In addition, since the GPS system is controlled by the US Department of Defense, its security and availability are under consideration for other countries and organizations.
To free the synchronization from the satellite communication, a plurality of synchronization methods have been proposed. Among them, IEEE 1588 is proposed to synchronize real-time clocks in the nodes of a distributed system that communicate using a network. It enables precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication, local computing and distributed objects. The clocks communicate with each other over a communication network. The protocol generates a master-slave relationship among the clocks in the system. The clock of a slave device is synchronized to the clock of its master device, and all clocks ultimately derive their time from a clock known as the grandmaster clock.
IEEE 1588 protocol eliminates the needs of communication to a satellite system and has been extensively investigated to be used in all kinds of networks. It can be either used to replace some GPS or as a complementation when GPS is not detected. For example, for Femto-cell, synchronization by means of IEEE 1588 protocol will be more reliable than using GPS since GPS signal might be too small to be detected.
In the IEEE 1588 protocol, each slave synchronizes to its master using Sync, Follow_Up, Delay_Req, and Delay_Resp messages. The ‘sync’ message is sent periodically (2 seconds in default) from the master device to the slave device to update the clock information at the slave device.
When the slave clock is accurate, the frequent transmission of sync message will waste the communication bandwidth in the network and also consume the calculation power in the slave device to perform corresponding operations. On the other hand, when the slave clock is not accurate or stable, it may result in a relatively large time offset between the master clock and the slave clock.